It's defining a variable named x of type vector<int> and using the value of corners to initialize it. When you use an integer to initialize a vector, that integer determines the number of elements the vector has.

A vector is based on a C++ system called a "template". You don't have to worry about exactly what a template is and how they work at this point, I'm sure you'll learn soon enough. In absolute simplest terms, a template allows you to define a class or function one (1) time and make it work for virtually any dataType without having to re-write it.

A vector's declaration includes this dataType as what is known as a "template argument" (the part between '<' and '>'). Thus, the general form of the declaration of a vector can be described as:

vector< elementDataType > vectorName(vectorSize)

"I want to create a vector of elementDataType objects. I want to call the vector "vectorName" and it will have "vectorSize" elements in it."

EDIT:
Oops... What arkoenig said too...
I really didn't want to mention it to prevent confusion, but there are actually several ways of defining a vector. The methods mentioned in the thread are probably the 2 most-common ways.